Semi-submersible vessel

ABSTRACT

A semi-submersible vessel is disclosed in which a side wall which is continuous in the horizontal direction connects an underwater buoyant pontoon hull to an operating deck located above the water surface. In operation the side wall is subject to moderate cyclic fatigue stresses only, thereby increasing the useful life of the vessel.

BACKGROUND OF THE INVENTION

This invention relates to a semi-submersible vessel.

Many designs of semi-submersible floating platforms used for drilling orother offshore operations are based on the use of three or more verticalbuoyant columns to support a deck at a safe distance above sea level,their bottom ends being attached to underwater chambers forming pontoonsto provide additional buoyancy.

The operating deck is designed for carrying equipment and livingquarters and may contribute to the rigidity of structure. The deck thenforms a strong box structure enclosing a number of tween decks andintegrated with the tops of the columns. Columns and bracings betweencolumns are subject to wave and current forces and their points ofattachment are susceptible to material fatigue.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide an offshore platformwhich does not rely on columns and bracings to support an operating deckand which displays superior strength and fatigue characteristics.

A vessel according to the present invention comprises a side wall whichis continuous in a horizontal direction, which wall is attached to anunderwater pontoon hull at its lower end and supports an operating deckat its upper end.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is an elevation view of a semi-submersible vessel according tothe invention;

FIG. 2 is a horizontal section of the side wall of the semi-submersiblevessel of FIG. 1, taken at line 2--2 therein; and

FIG. 3 is a horizontal cross-sectional view of the configuration of theoperating deck of the vessel shown in FIG. 1 taken at line 3--3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The vessel shown in the drawing comprises a horizontally continuous wall1 to support the operating deck 2. The wall 1 may be circular in planview or multifaceted. The bottom of wall 1 is attached to a pontoon hull3 of generally circular or multifaceted cross-section and with a similarplan shape to the wall. The operating deck 2, side wall 2 and pontoonhull 3 preferably have the shape of ring-like polygons surrounding asubstantially prismatic central opening 15. Said polygons and prismaticcentral opening 15 preferably have a common substantially vertical axisof symmetry 14.

The wall 2 comprises a series of buoyant compartments 4. Between thebuoyant compartments are perforations 5 passing through the wall toallow passage of water and waves into and from the central opening 15.These perforations 5 are necessary to reduce the wave loadings andimpact forces acting on the wall and assist in reducing hull motions.The number, shape and spacing of the perforations will be designed tosuit operational and constructional requirements but a permeability of30% is typical.

The wall 2 is self supporting and does not rely on the deck structure toprovide rigidity or require cross bracings. The design of the operatingdeck may therefore be designed to suit operational requirements and onearrangement shown in FIG. 3 comprises a flat annular deck 6 supported oncantilever beams 7 fixed to the top of the wall. Other deckconfigurations are possible to suit particular operations, including afully plated deck spanning the entire central area from wall to wall. Astrong box deck can be used to contribute to the overall structuralstrength of the vessel.

The underwater pontoon hull 3 and side wall are divided into ballasttanks 8 connected to pumping machinery to allow the draft to be altered.Internal access to these tanks is provided :or inspection purposes.Tanks in the walls 9 are also used for fuel, potable water and otherconsumable supplies. The vessel is held in position by catenary mooringlines 10 attached to the side wall and secured to seabed anchors (notshown).

The height of the wall is such that the waterline 11 is locatedapproximately half the height of the wall guaranteeing that theoperating deck is located well above the waves and that the pontoon isimmersed to such an extent that it does not emerge during the passage ofthe largest waves.

The essential feature of all semi-submersibles is a small water planearea symmetrically placed in relation to the axis of symmetry of thevessel. In the semi-submersible vessel according to the invention, theaverage distance from the side wall 2 to the vertical axis of symmetry14 is chosen large enough to provide the large water plan inertianecessary t0 produce good stabiliy characteristics.

The vessel shown is intended to be used as an oil production vessel andis provided with flexible riser pipes 12 to conduct produced fluids fromwellhead to vessel. A hydrocarbon processing plant 13 is situated on theoperating deck and separated oil and gas are removed from the vesseleither by flexible risers to a seabed pipeline or by tanker.Accommodation for personnel is provided in a deck house 15 separatedfrom the hazardous areas associated with hydrocarbon operations. Surplusgas can be flared using special burners mounted on a tower or boom and ahelicopter platform is provided for personnel transfer. Facilities areprovided to store the vessel from a supply boat stationed adjacent tothe vessel.

The embodiment described above and shown on the drawings is merely anexample of the invention. The components forming part of the platformmay be varied within the scope of the appended claims, which define astructure which is safe with respect to strength and stability, even indamaged conditions. The design permits easy inspection of the underwaterparts and is especially suited for use in waters where icedrift isexpected.

Other modifications, changes and substitutions are intended in theforegoing disclosure and in some instances some features of theinvention will be employed without a corresponding use of otherfeatures. Accordingly, it is appropriate that this appended claim beconstrued broadly and in a manner consistent with the spirit and scopeof the invention herein.

What is claimed is:
 1. A semi-submersible vessel comprising an inhorizontal direction continuous side wall attached at the lower endthereof to an underwater pontoon hull and supporting at the upper endthereof an operating deck, wherein the side wall and pontoon hull haveeach a substantially vertical axis of symmetry and surround each acentral opening which is concentric to said axis.
 2. The vessel of claim1 wherein the side wall is substantially tubular shaped and is mountedon and coaxially to a substantially ring-shaped pontoon hull.
 3. Thevessel of claim 1 wherein structural members subdivide the wall andpontoon hull into tanks to provide buoyancy and for storage ofconsumable supplies and ballast water.
 4. The vessel of claim 1 whereinthe side wall has a positive buoyancy and is perforated to allow passageof water and waves into and from said central opening.
 5. The vessel ofclaim 4 wherein the side wall is perforated by an array of generallyrectangular perforations.
 6. The vessel of claim 1 wherein the side wallforms, in a horizontal plane of cross-section, a surface of revolutionand extends vertically above a pontoon hull which forms a similarsurface of revolution.
 7. The vessel of claim 6 wherein said surface ofrevolution is a polygon.
 8. The vessel of claim 7 wherein said centralopening extends vertically and has a substantially prismatic shape.
 9. Asemi-submersible vessel comprising:an underwater pontoon hull; a sidewall which is continuous in a horizontal direction attached at the lowerend thereof to the underwater pontoon hull; and an operating deck whichis supported by the upper end of the side wall; wherein the side walland the pontoon hull are each symmetrical about a substantially verticalaxis of symmetry and each surrounds a central opening which isconcentric to said axis of symmetry.
 10. A semi-submersible vessel inaccordance with claim 9 wherein the side wall is substantially tubularshaped and the pontoon hull is substantially ring-shaped.
 11. Asemi-submersible vessel in accordance with claim 9 further comprising aplurality of structural members subdividing the interior of the sidewall to define a plurality of tanks.
 12. A semi-submersible vessel inaccordance with claim 11 wherein the tanks are suitable to providebuoyancy and for storage of consumable supplies and ballast water.
 13. Asemi-submersible vessel in accordance with claim 9 wherein the side wallhas a positive buoyancy and is perforated to allow passage of water andwaves into and from said central opening.
 14. A semi-submersible vesselin accordance with claim 13 wherein the side wall is perforated by anarray of generally rectangular perforations.
 15. A semi-submersiblevessel in accordance with claim 9 wherein the side walls form a surfaceof revolution in a horizontal plane of cross-section which extendsvertically above the pontoon hull which forms a similar surface ofrevolution in a horizontal plane of cross section.
 16. Asemi-submersible vessel in accordance with claim 15 wherein the surfacesof revolution for the side wall and the pontoon hull is a polygon.
 17. Asemi-submersible vessel in accordance with claim 16 wherein the centralopening has a substantially prismatic shape.
 18. A semi-submersiblevessel comprising:a substantially ring-shaped underwater pontoon hullwhich is symmetrical about a vertical axis of symmetry; a substantiallytubular shaped side wall attached at its lower end to the underwaterpontoon hull and defining a central opening which is concentric with thevertical axis of symmetry, said side wall having a positive buoyancy andbeing continuous in a horizontal direction and further defining aplurality of perforations which allow passage of water and waves intoand from said central opening; a plurality of structural memberssubdividing the interior of the side wall to define a plurality oftanks; and an operating deck which is supported by the upper end of theside wall.